Sen. Joe Manchin, D-W.Va. listens at left, as Sen. Patrick Toomey, R-Pa. announce Wednesday on Capitol Hill in Washington that they have reached a bipartisan deal on expanding background checks to more gun buyers.

WASHINGTON — Two key senators have reached a bipartisan deal on expanding background checks to more gun buyers, a Senate aide and lobbyist said Wednesday, an agreement that could build support for President Barack Obama’s drive to curb firearms violence in the wake of the elementary school shootings in Connecticut.

Sens. Joe Manchin, D-W.Va., and Patrick Toomey, R-Pa., planned to announce their compromise later Wednesday morning. Subjecting more firearms purchases to federal background checks has been the chief goal of Obama and gun control supporters, who promote the system as a way to prevent criminals and other potentially dangerous people from getting the weapons.

Meanwhile, the Senate is ready for an opening vote on restricting guns as Majority Leader Harry Reid, D-Nev., set a roll call for Thursday on starting consideration of the firearms legislation. The background check deal makes it even likelier that Democrats will win enough Republican support to thwart an effort by conservatives and Senate Minority Leader Mitch McConnell, R-Ky., to block consideration from even starting.

The background check deal would expand the system to cover all commercial sales, such as at gun shows and online. Records of the transactions would have to be kept by licensed gun dealers, the same system used currently.

Private transactions that are not for profit, such as those between relatives, would be exempt from background checks.

Currently, background checks are required only for sales through licensed gun dealers.

The aide and lobbyist spoke on condition of anonymity to describe private talks.

The administration was continuing its effort to pressure Congress on gun control on Wednesday as first lady Michelle Obama planned to visit a Chicago high school where authorities say 29 current or former students have been shot in the past year. Eight of them died.

The background check deal and the Senate’s scheduled threshold vote on the gun bill were a boost for advocates battling for firearms restrictions in the wake of December’s shootings that killed 20 first-graders and six staffers at an elementary school in Newtown, Conn.

“I hope Republicans will stop trying to shut down debate and start engaging in the tough issues we were sent to Washington to tackle,” Reid said.

Manchin and Toomey are among their parties’ most conservative members and a deal could make it easier for some hesitant senators to support the background check measure, at least for now.

Even so, the ultimate fate of gun legislation remains unclear, clouded by opposition from many Republicans and moderate Democrats in the Democratic-led Senate and the Republican-run House. Many critics say the proposal would violate the Second Amendment right to bear arms and burden law-abiding gun owners.

Sen. Chuck Schumer, D-N.Y., who led an earlier unsuccessful effort to strike a bipartisan background check deal, is backing the compromise after changes were made from an initial version of the deal between Manchin and Toomey, according to a Senate aide who spoke on condition of anonymity to describe details of the talks.

The changes included eliminating language that would have required states to recognize permits to carry concealed weapons issued by other states and eliminating language that would have limited background checks to sellers who sell at least five guns annually, said the aide.

Some Republicans might vote to begin debate on the legislation but eventually oppose the measure on final passage. Other parts of Obama’s gun effort already seem likely to face defeat, including proposed bans on assault weapons and high-capacity ammunition magazines.

The gun legislation Reid wants the Senate to debate would extend the background check requirement to nearly all gun sales. Reid would try to replace that language with the Manchin-Toomey compromise once debate begins, a move that would require a vote.

The overall gun bill also tightens federal laws against illegal gun sales and slightly increases federal aid for school safety.

Thirteen conservatives have signed a letter saying they will block consideration of the measure, and McConnell said he will back that move. That will force Democrats to round up 60 votes to overcome the conservatives.

At least eight Republicans have said they want to begin debate or have indicated a willingness to consider it, a number that would be expected to grow if the background check agreement proves successful.

Some moderate Democrats were remaining noncommittal and could oppose opening the gun debate. There are 53 Senate Democrats and two independents who lean Democratic.

Amid the maneuvering, relatives of some Newtown victims are lobbying to support gun curbs. And Obama has been calling senators from both parties to push for the gun bill.

“People should listen to what we have to say and move the debate forward,” said Mark Barden, who lost his 7-year-old son, Daniel. “It’s not just about our tragedy. Lots of kids are killed every day in this nation. We have to help lead the change.”

The National Rifle Association opposes Obama’s effort and is urging its members — it claims nearly 5 million — to tell lawmakers of their opposition.

In GOP-heavy Louisiana, the NRA asked members to contact Democratic Sen. Mary Landrieu, who is seeking re-election next year. “The future of our Second Amendment rights are at stake,” the mailing said.

Counteracting that has been an effort by Mayors Against Illegal Guns, one of whose leaders is billionaire New York Mayor Michael Bloomberg. It said it will keep track of key gun-related congressional roll calls and make the information available to voters and contributors — a tactic long used by the NRA and other groups.